In the oil industry the term "alkylate" generally refers to the product of the alkylation of C.sub.2 -C.sub.6 olefins with C.sub.4 -C.sub.6 alkanes (generally known as motor fuel alkylation). The product is a mixture of alkanes. The main use of alkylate is as a component in motor fuel and thus, it is desirable that the alkylate contain branched paraffins and especially trimethylpentanes, since these paraffins have a high octane number. Although alkylate was an important component when anti-knock additives were used, its importance has increased significantly owing to the phaseout of lead in gasoline. Finally, increased demand of alkylate is owing to the decreased use of butane because butane evaporates readily, especially in warm weather, contributing to smog formation.
The alkylation of C.sub.2 -C.sub.6 olefins is generally catalyzed by strong acids such as sulfuric and liquid hydrogen fluoride. Commercially, HF has been favored at least in part because of the relative ease of HF regeneration. An overview of HF-catalyzed alkylation is presented by B. R. Shah in "Handbook of Petroleum Refining Processes", R. A. Meyers, editor, McGraw-Hill Book Company, 1986, pp 1-3 through 1-28. Briefly, the HF-catalyzed alkylation process is carried out as follows. Generally, olefinic and isobutane feedstocks are combined and mixed with HF in an alkylation reaction zone. The reactor effluent is separated into the desired alkylate, acid, and other light gases which are predominantly unreacted isobutanes. The HF is either recycled to the reactor directly or regenerated in whole or in part prior to its being recycled to the reactor. Unreacted isobutane is also recycled to the reactor and the alkylate is blended in with the motor fuel pool.
However, recently there has been heavy environmental pressure to eliminate the use of HF as a catalyst. HF (hydrofluoric acid) is classified as an acutely hazardous material and in Southern California the Board of the South Coast Air Quality Management District has required that the use of HF in alkylation be phased out by Jan. 1, 1998. Accordingly, there is a strong need to find a catalyst which can replace HF as an alkylation catalyst. It is also desirable to have a solid acid as the catalyst since this would allow the use of a fixed bed system which is generally preferred over a liquid/liquid system by the petroleum refining industry.
Although solid acid catalysts are known in the art, they have serious drawbacks which have precluded them from being used commercially for the alkylation of C.sub.2 -C.sub.6 olefins. For example, U.S. Pat. No. 2,999,074 discloses an alkylation catalyst comprising a refractory inorganic oxide support having dispersed thereon platinum and the reaction products between one or more of the metal halides (those that are active as Friedel-Crafts catalysts) and the refractory inorganic oxide support, e.g., alumina. The platinum metal provides hydrogenation activity for the olefins. One disadvantage of this type of catalyst is that it is too acidic, which results in the catalyst not only catalyzing the alkylation of the olefins but concurrently cracking some of the various alkylate products. Another disadvantage of these types of catalyst is that they are insufficiently selective, again probably arising from its "excess" acidity. By selectivity is meant the extent of monoalkylation versus multiplealkylation. For example, it is most desirable that the olefin react only with the alkanes initially in the feedstocks. However, in practice some of the alkylation products formed also subsequently react with the olefin in the feedstock to form secondary alkylation products. For example, if the feedstock contains only butanes and butenes, the primary alkylation product would be a C.sub.8 paraffin, while secondary alkylation products would be C.sub.12, C.sub.16, and so on. Since this type of catalyst has cracking activity, the secondary alkylation products can be cracked to lower molecular weight hydrocarbons. Finally, another side reaction which should be avoided is oligomerization which leads to inefficient olefin consumption.
Applicants have discovered a novel solid alkylation catalyst which has improved selectivity and a much longer life than previously known catalysts. The catalyst of the present invention comprises a particulate refractory inorganic oxide support having dispersed thereon at least one Group VIII metal such as platinum, a metal cation such as potassium and the reaction product of a metal halide, e.g., aluminum chloride, and bound surface hydroxyl groups on said refractory inorganic oxide. The catalyst is characterized in that at least 40 percent of the Group VIII metal present on the catalyst is located within a 100-300 micrometer external layer of the catalyst particle, while the metal cation is uniformly distributed throughout the particle. The increased life of the instant catalyst makes the use of a solid bed alkylation catalyst commercially feasible.
It should be pointed out that U.S. Pat. No. 5,017,541 discloses a catalyst which is used for isomerization and which is composed of a support, a Friedel-Crafts metal halide and a surface layer platinum group metal. However, this reference does not mention alkylation, nor the use of a metal cation such as potassium.